A Pimco fund is one of only three actively managed exchange-traded funds that top $1 billion in assets

At least one actively managed exchange-traded fund is now in the crosshairs of regulators, but the sector accounts for just a sliver of the $2.1 trillion ETF industry.

On Monday, bond giant Pacific Investment Management Co. disclosed that its $2.55 billion Pimco Total Return Active ETF is facing regulatory scrutiny over how it valued its assets. The fund is the second-largest actively managed exchange-traded fund and one of just three active ETFs to have more than $1 billion in assets.